Frequency synchronization for non-terrestrial cellular wireless communication networks

ABSTRACT

The invention provides a method and an architecture for deploying non-terrestrial cellular network base stations, so as to enable cellular network coverage in remote areas, where no fixed infrastructure is available. The proposed methods allow for efficient power management at the terminal devices that need to synchronize to the airborne or spaceborne cellular base stations. This is particularly important for IoT devices, which have inherently limited power are computing resources.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.17/227,218, filed Apr. 9, 2021, which is a continuation of InternationalApplication No. PCT/EP2019/077813, filed Oct. 14, 2019, which claims thebenefit of European Application No. 19191615.4, filed Aug. 13, 2019, andEuropean Application No. 18200058.8, filed Oct. 12, 2018 the entiredisclosures of which are hereby incorporated by reference herein for allpurposes.

TECHNICAL FIELD

The present invention lies in the field of cellular wirelesscommunication systems. In particular, the invention relates to cellularwireless communication systems comprising non-terrestrial networkinfrastructure, such as a cellular base station hosted on a satellitepayload.

BACKGROUND OF THE INVENTION

Cellular wireless communication networks are nowadays widely availablein developed and mainly urban environments. A ground-based networkallows a user equipment, such as a telephone, smartphone or personalcomputer, to establish a data communication link with a data networksuch as the public Internet via a base station that manages thegeographical network cell in which they evolve.

Several cellular network standards have been deployed based on GSMtechnology, UMTS/3G, LTE/4G and 5G networking technology. Crucially,when no cellular network infrastructure is available in a givengeographic area, no wireless data communication is available for anyusers or device in that area. In remote areas, the construction ifnetwork infrastructure is often difficult and overly costly given a lowpopulation or device density.

The Internet of Things is a paradigm in which devices such as objects orsensors are able to enter into communication with a remote networkbackend, such as a data center or data processing server. Thetransmission of data from an IoT device is often not delay critical.However, a reliable communication link to the network backend needs tobe established, at least intermittently. IoT devices may for example bedeployed on maritime vessels, or in remote areas. However, in suchareas, cellular network access is often not provided by the traditionalfixed networking infrastructure. Such devices are usuallybattery-powered, so that the available transmit power is limited at anypoint in time.

The deployment a non-terrestrial cellular wireless communicationnetwork, involving a partly airborne/spaceborne network architectureappears to be an interesting solution for providing cellular datanetwork coverage to the requisite remote areas. However, at the time ofwriting there is no solution in the state of the art which would enablecellular access from a piece of user equipment having low availablepower, via a non-terrestrial piece of network infrastructure equipment.Today's communication standards have indeed been designed with unlimitedpower as a prerequisite for the infrastructure, as well as low delaysand stationary location with respect to the user equipment.

Technical Problem to be Solved

It is an objective to present method and device, which overcome at leastsome of the disadvantages of the prior art.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, a control method fora terminal device in a non-terrestrial cellular data communicationnetwork is provided. The network comprises at least one airborne orspaceborne base station moving along a flight trajectory, for connectingthe terminal device to said network. The method is remarkable in that itcomprises the following steps:

a) providing flight trajectory data for said base station in a memoryelement of said terminal device;

b) providing terminal location data in a memory element of said terminaldevice;

c) using a data processing unit, determining at least one time slotduring which a wireless communication channel between said terminaldevice and said base station is estimated to be available, based on saidflight trajectory data and on said terminal location data;d) using a data processing unit, scheduling a data reception ortransmission between said terminal device and said base station duringthe determined time slot.

Preferably, the step of scheduling a data reception transmissioncomprises switching the terminal device's state using said dataprocessing unit from a first state, in which the device is not capableof receiving and/or transmitting data, to a second state, in which thedevice is capable of receiving and/or transmitting data using a datareception and transmission unit. Preferably, the first state ischaracterized by a power consumption of the terminal device that islower than the power consumption of the terminal device when the latteris in the second state. In the first state, the terminal device maypreferably be configured to carry out other tasks, such as sensing orprocessing data, rather than receiving/transmitting data to/from anairborne/spaceborne base station of the cellular network.

Preferably, said data processing unit is a data processing unit of saidterminal device. The data processing unit may preferably comprise acentral processing unit, CPU, operatively coupled to a memory elementcomprising any of a solid-state drive, SSD, hard disk drive, HDD, randomaccess memory, RAM, or any other known data storage element. The datareception and data transmission means may preferably comprise a cellularnetworking interface, comprising a receive antenna and a transmitantenna, as well as any required subsystems thereof, for operativelyconnecting said terminal device to the non-terrestrial cellular datacommunication network. Alternatively, the terminal device may haveremote access, by means of a data communication channel, to said dataprocessing means. The data processing unit may be provided by set ofdistributed computing devices configured for providing the describedfunctionality.

Preferably, the step of providing flight trajectory data may comprise,at the base station, transmitting said flight trajectory data to saidterminal device, and at the terminal device, using a data receptionunit, receiving said flight trajectory data from said base stationthrough a wireless data communication channel.

Said data may preferably be broadcast to any available terminal devicesform said base station.

The base station may preferably receive said flight trajectory data froma ground-based network node. Alternatively, the base station maycomprise a memory element on which said flight trajectory data has beenpre-stored.

The flight trajectory data may preferably comprise an identifier of saidbase station, ephemeris data, altitude data, velocity data or anycombination thereof.

Preferably, the flight trajectory data may comprise flight trajectorydata describing the trajectories of a plurality of airborne orspaceborne base stations, together with their respective identifiers.

The terminal device may preferably compute, using said data processingunit and based on said flight trajectory data, estimate values for saidbase station, comprising any of a future position, elevation, velocity,Doppler shift, Doppler drift, propagation delay, derivatives of saidDoppler shift or propagation delay, or any combination thereof, andstores these estimate values in a memory element.

The non-terrestrial cellular data communication network may preferablycomprise at least two airborne or spaceborne base stations. The step ofproviding terminal location data may further preferably comprise theadditional step of estimating the terminal device's location based ondetected properties of signals received at the terminal device from saidbase stations.

In accordance with another aspect of the invention, a control method fora terminal device in a non-terrestrial cellular data communicationnetwork is provided. The method is remarkable in that it comprises thestep of estimating the terminal device's location based on detectedproperties of at least three signals received at the terminal devicefrom different positions taken by at least one base station. Preferablythe three positions may correspond to three fly-overs of the basestation over the terminal device.

Preferably, said at least one airborne or spaceborne base stations maybe configured for transmitting a reference timing signal to saidterminal device, and the non-terrestrial cellular data communicationnetwork may preferably comprise a location service node storinginformation describing each base station's respective flight trajectory,and wherein the step of providing terminal location data furthercomprises the following preliminary steps:

-   -   at the terminal device, accumulating over time the respective        arrival times of the reference timing signal received from at        three different positions taken by at least one base station;    -   computing, from said arrival times, at least two arrival time        differences with respect to one reference arrival time; the        reference arrival time may be preferably selected among said        accumulated arrival times;    -   transmitting, at the terminal device, said computed differences        to said location service node, through one of said base        stations;    -   at said location service node, receiving said computed        differences, computing a location estimate of said terminal        device using said computed differences and said stored        information describing each base station's respective flight        trajectory, and transmitting said location estimate to said        terminal device, through one of said base stations; and    -   at the terminal device, receiving said location estimate from        one of said base stations and storing it in a memory element.

The three different position of the base stations preferably correspondto three different reception times of the reference timing signal at theterminal device, as the position of the base stations evolves along itstrajectory.

Alternatively, the terminal device may transmit said measured arrivaltimes to a network node, for example to said location service node,which is further configured for computing said arrival time differences.

Preferably, the non-terrestrial cellular data communication network maycomprise a plurality of airborne or spaceborne base stations configuredfor transmitting a common synchronized reference timing signal, and thestep of accumulating said arrival times may comprise the reception ofthe reference timing signal at the terminal device from at least two orthree of said airborne or spaceborne space stations. The arrival timesmay be accumulated from a plurality of base stations that are within theterminal device's line of sight at the same time, or from a single basestation changing its position relative to the terminal device over time,or from a plurality of base stations changing their positions relativeto the terminal device over time.

Preferably, the terminal device may further receive, during saidscheduled time slot, a synchronization signal from said base station,the synchronization signal carrying data indicating a transmissionfrequency and timing information, which are required for the terminaldevice to synchronize future data transmission and/or data receptionto/from said base station.

In accordance with another aspect of the invention, a control method fora terminal device in a non-terrestrial cellular data communicationnetwork is provided. The network comprises at least one airborne orspaceborne base station moving along a flight trajectory, for connectingthe terminal device to said network. The method is remarkable in that itcomprises the following steps:

-   -   at the terminal device, receiving a synchronization signal from        said base station, the synchronization signal carrying data        indicating a transmission frequency and timing information,        which are required for the terminal to synchronize future data        transmission and/or data reception to/from said base station.

Preferably, the terminal device may compute, using a data processingunit, an observed Doppler shift based on the receiving frequency forsaid synchronization signal and on the transmission frequency indicatedtherein, and the terminal device may pre-emptively compensate saidtransmission frequency by a frequency compensation value during asubsequent data transmission to said base station, said frequencycompensation value taking into account any of said observed Dopplershift. Said computation may alternatively be done at a remote processingunit, to which the terminal device has access.

Preferably, said frequency compensation value may further take intoaccount any of said estimated Doppler shift values, Doppler drift, aderivative of the observed Doppler shift, or any combinations thereof,at the time of said subsequent data transmission.

The terminal may further preferably compute, using a data processingunit, an observed time shift based on the reception time of saidsynchronization signal and on the timing information indicated therein,and the terminal device may further pre-emptively compensate thescheduled time of transmission by a time compensation value during asubsequent data transmission to said base station, said timecompensation value taking into account said observed time shift, and/orany derivative thereof. Alternatively, said computation may be done at aremote processing unit to which the terminal device has access.

Preferably, said time compensation value may further take into account aconstant timing offset that is a function of the base station'sposition. Preferably said time compensation value may depend on the basestation's altitude.

Preferably, a base station is may be estimated to be available, if itselevation above said terminal device is estimated to be above apredetermined elevation threshold value. Said threshold value may be inthe range between 45° and 70°, it may further preferably be of 60°.

The base station may preferably be an airborne base station comprisingany of a high-altitude platform, HAP, a drone, or an airplane.

The base station may preferably be part of a fleet of interconnectedairborne base stations.

Preferably, the base station may be a spaceborne base stationscomprising a Low Earth Orbit, LEO, Middle Earth Orbit, MEO orGeostationary Orbit, GEO satellite.

Said satellite may preferably be part of a constellation of satellites,wherein a plurality of satellites are interconnected base stations ofsaid non-terrestrial cellular data communication network.

Preferably, interconnect base stations exchange data describingrespectively connected terminal devices with each other, in order tofacilitate a handover between two base stations. Preferably, each basestation stores identifiers of currently neighbouring base stations in amemory element, and updates these periodically. Preferably, each basestation stores data describing respectively connected terminal device ina memory element and updates these periodically.

Preferably, each airborne or spaceborne base station periodicallyupdates data describing the set of currently neighbouring airborne orspaceborne base stations. Said data preferably describes allneighbouring base station that may currently be interconnected.

The flight trajectory data may preferably comprise Two-Line-Element,TLE, data.

The terminal device may preferably be a user equipment or a ground-basedgateway node serving a plurality of user equipment.

In accordance with a further aspect of the invention, a terminal devicefor a non-terrestrial cellular data communication network is provided.The terminal device comprises a data transmission unit, a data receptionunit, a memory element for storing flight trajectory data of an airborneor spaceborne base station of said network, a memory element for storingit location data, and a processing unit, wherein the processing unit ifconfigured to

determine at least one time slot during which a wireless communicationchannel between said terminal device and said base station is estimatedto be available, based on said flight trajectory data and on saidterminal location data, and to

schedule a data reception or transmission between said terminal deviceand said base station during the determined time slot.

Preferably, the processing unit may further be configured to implementthe terminal device-based method steps in accordance with any aspect ofthe invention.

In accordance with another aspect of the invention, a base station for anon-terrestrial cellular data communication network is provided. Itcomprises a data transmission unit, a data reception unit, a memoryelement and a data processing unit, wherein the data processing unit isconfigured for transmitting data describing a projected or actual flighttrajectory of the base station.

The base station may preferably comprise a satellite, a drone, ahigh-altitude platform or an airplane.

In accordance with yet another aspect of the invention, anon-terrestrial cellular data communication system is provided. Thecommunication system comprises at least one terminal device inaccordance with aspects of the invention, and at least one airborne orspaceborne base station in accordance with aspects of the invention.

Preferably, the communication system may comprise a 4G NarrowbandInternet-of-Things communication system. Preferably, the airborne orspaceborne base station may comprise an implementation of thefunctionality of an eNb, evolved Node B node, in accordance with saidcommunication system.

In accordance with a further aspect of the invention, a computer programcomprising computer readable code means is provided, which when run on acomputer, causes the computer to carry out the terminal device basedsteps of the method according to aspects of the invention.

In accordance with another aspect of the invention, a computer programcomprising computer readable code means is provided, which when run on acomputer, causes the computer to carry out the base station based stepsof the method according to aspects of the invention.

In accordance with a final aspect of the invention, a computer programproduct comprising a computer-readable medium is provided, on which thecomputer program according to aspects of the invention is stored.

By using the methods in according with aspects of the invention, itbecomes possible to use an airborne or spaceborne base station, such asfor example an eNodeB type communication node implemented on asatellite, as an access point for a piece of user equipment having lowavailable power. This enables for example to connect Internet of Things,IoT, devices to a global network, without requiring the construction offixed cellular networking infrastructure. By using flight pathinformation describing the airborne/spaceborne base station'strajectory, as well as an estimation of its own location, the userequipment device is able to estimate when the base station will bewithin its line of sight, so that in the meantime power may be saved.Once the base station is within the device's line of sight, the devicemay synchronize to the base station in terms of transmission delay andDoppler drift, and pre-emptively compensate any delay and/or drift infollowing up-link transmissions, thereby improving the efficiency of thecommunication between the user equipment and the base station. Inaccordance with aspects of the invention, an estimated Doppler shiftand/or transmission delay, which is based on the available flight pathinformation with respect to the user equipment's own location, mayfurther be used when no measure of these values is available, or inorder to refine any measured Doppler shift or transmission delay for asubsequent pre-compensated uplink transmission to the base station. Byavoiding retransmissions due to loss of synchronization between the userequipment and the base station, this approach is able to save power onboth the user equipment device and on the airborne/spaceborne basestation, which as limited resources as well. In accordance with otheraspects of the invention, in which a fleet of airborne or spacebornebase stations, comprising for example a constellation of interconnectedsatellites is available, a method of estimating the user equipment's owngeographical location is further provided, which is useful if no othergeolocation services are available.

BRIEF DESCRIPTION OF THE DRAWINGS

Several embodiments of the present invention are illustrated by way offigures, which do not limit the scope of the invention, wherein:

FIG. 1 illustrates a known radio interface protocol architecture;

FIG. 2 illustrates a known protocol sequence between a terminal deviceand a ground based base station of a cellular LTE/NB-IoT network;

FIG. 3 illustrates a known functional device architecture of a terminaldevice of a cellular LTE/NB-IoT network;

FIG. 4 illustrates a known TDOA positioning algorithm for terrestrialcellular networks;

FIG. 5 illustrates a known location service procedure in a terrestrialcellular network;

FIG. 6 illustrates the main steps of a method in accordance with apreferred embodiment of the invention;

FIG. 7 illustrates a non-terrestrial cellular communication network inaccordance with a preferred embodiment of the invention;

FIG. 8 illustrates a store and forward architecture for anon-terrestrial cellular communication network;

FIG. 9 illustrates a real-time non-terrestrial cellular communicationnetwork with inter-satellite link;

FIG. 10 illustrates a femtocell architecture for a non-terrestrialcellular communication network;

FIG. 11 illustrates the main steps of an algorithm for propagatingflight trajectory data in time;

FIG. 12 illustrates the main steps of an algorithm for predictingwake-up times of a terminal device, in accordance with a preferredembodiment of the invention;

FIG. 13 illustrates a protocol sequence for a non-terrestrial cellularcommunication network, in accordance with a preferred embodiment of theinvention;

FIG. 14 illustrates a function architecture for a terminal device in anon-terrestrial cellular communication network, in accordance with apreferred embodiment of the invention;

FIG. 15 illustrates a spaceborne base station and a terminal device of anon-terrestrial cellular communication network, in accordance with apreferred embodiment of the invention;

FIG. 16 illustrates the operation of a terminal device, when itslocation is known, in accordance with a preferred embodiment of theinvention;

FIG. 17A illustrates a known frequency raster search method;

FIG. 17B illustrates a frequency raster search method in accordance witha preferred embodiment of the invention;

FIG. 18 illustrates the operation of a terminal device, when itslocation is unknown, in accordance with a preferred embodiment of theinvention;

FIGS. 19A and 19B illustrate timing alignment of an uplink transmission(a) without timing advance and (b) with timing advance in accordancewith a preferred embodiment of the invention; and

FIG. 20 illustrates an example scenario of the system operation inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

This section describes aspects of the invention in further detail basedon preferred embodiments and on the figures. The figures do not limitthe scope of the invention. Unless otherwise stated, similar conceptsare referenced by similar numerals across embodiments. For example,references 100, 200, 300 and 400 each refer to a non-terrestrialcellular data communication network, in accordance with a first, secondthird and fourth embodiment of the invention.

A set of functional and architectural modifications are proposed toenable cellular wireless technology to operate over non-terrestrialnetworks. The disclosed embodiments are related generally to the fieldof cellular wireless communications, in particularly for its operationover a constellation or a group of non-terrestrial base stations,including but not limited to drones, airplanes, High Altitude PlatformStation, HAPS, and satellite systems.

By way of a non-limiting example, the 3GPP cellular standard is designedto operate using terrestrial networks. In the of development anyterrestrial standard, it is always assumed that the radio and corenetworks are never short of resources in terms of both processorcapacity and power requirements. However, for such a standard to operateover non-terrestrial networks, in addition to power constraints andprocessor capacity limitations, the channel model, pathloss and otherfunctional conditions for operations such as Doppler and propagationdelay add to the complexity of the system design.

Therefore, the present invention offers solution to the problem byproposing a set of functional and architectural modifications to thesystem that will enable the successful and efficient operation ofcellular wireless communication protocols over non-terrestrial networks.While the proposed methods may be useful on their own, they may also becombined with each other to provide an overall improved method enablingefficient communication in a non-terrestrial cellular data communicationnetwork.

Existing Communication Standards

The radio interface for 3GPP LTE/NB-IoT release 13 covers the interfacebetween the User Equipment, UE, and the network. The correspondingstandards document is publicly available for example from www.etsi.orgat the following path:/deliver/etsi_ts/136100_136199/136104/13.04.00_60/ts_136104v130400p.pdfand its content is hereby incorporated by reference in its entirety. Theradio interface is composed of the Layer 1, 2 and 3. The TS 36.200series describes the Layer 1 (Physical Layer) specifications. Layers 2and 3 are described in the 36.300 series.

FIG. 1 shows the E-UTRA radio interface protocol architecture around thephysical layer (Layer 1). The physical layer interfaces the MediumAccess Control, MAC, sub-layer of Layer 2 and the Radio ResourceControl, RRC, Layer of Layer 3. The circles between differentlayer/sub-layers indicate Service Access Points, SAPs. The physicallayer offers a transport channel to MAC. The transport channel ischaracterized by how the information is transferred over the radiointerface. MAC offers different logical channels to the Radio LinkControl, RLC, sub-layer of Layer 2. A logical channel is characterizedby the type of information that is transferred, rather than physicalcharacteristics of the channel.

The 3GPP 4G LTE/NB-IoT protocol sequence, as shown in FIG. 2 , isbroadly classified as:

-   -   1. Synchronization    -   2. Broadcast and system information transmission    -   3. Random Access    -   4. RRC Signaling    -   5. User data transmission    -   6. Connection release

While this nomenclature is specific to the 4G standard, the principlesare similar in all cellular protocols. The corresponding known devicearchitecture that enables the connectivity is shown for reference inFIG. 3 .

Generally an execution of a known positioning method, independent of themethod being based on satellite or mobile radio signals, consist ofthree steps:

1. Providing initial assistance and information for position estimation.

2. Execution of certain measurements and reporting of measurementresults.

3. Position estimation based on measurement results.

The supported positioning methods in cellular based positioning servicesrely on a high-level network architecture shown. For example, as one ofthe design goals for LTE/NB-IoT was to decentralize everything, thenetwork architecture has been defined so that it is generallyindependent from the underlying network. There are three main elementsinvolved in the process, the Location Service Client, LCS, the LCSServer, LS, and the LCS Target. A client, i.e., the requesting service,is in the majority of the cases installed or available on the LCStarget. This service obtains the location information by sending arequest to the server. The location server is a physical or logicalentity that collects measurements and other location information fromthe device and base station and assists the device with measurements andestimating its position. The server basically processes the request fromthe client and provides the client with the requested information andoptionally with velocity information.

There are generally two different possibilities for how the device(client) can communicate with the location server. There is the optionto do this over the user plane (U-Plane), using a standard dataconnection, or over the control plane (C-Plane). In the control planethe E-SMLC (Evolved Serving Mobile Location Center) is of relevance aslocation server, where for the user plane this is handled by the SUPLLocation Platform. SUPL stands for Secure User Plane Location and is ageneral-purpose positioning protocol defined by the Open Mobile Alliance(OMA). Both E-SMLC and SLP are just logical entities and can be locatedin one physical server.

One such mechanism is the Observed Time Difference Of Arrival, OTDOA,technique. In this technique the UE uses a multilateration method tomeasure the time of arrivals, TOA, of a particular reference signal, thePositioning Reference Signal, PRS, from multiple base-stations. The UEuses one base station as the reference base station and subtracts theTOA of the PRS of it from the several neighbor base-stations.

Geometrically, each time (or range) difference determines a hyperbola,and the point at which the hyperbolas intersect is the desired UElocation. This is illustrated in FIG. 4 . At least three timingmeasurements from geographically dispersed base-stations with goodgeometry are needed to solve for two coordinates (x,y orlatitude/longitude) of the UE.

For this, in case of LTE (Long Time Evolution), a set of procedures arefollowed in the LTE at a known protocol level which is illustrated inFIG. 5 . This is as follows:

1a. Either: the UE requests some location service (e.g. positioning ordelivery of assistance data) to the serving MME at the NAS level.

1b. Or: some entity in the EPC (evolved packet core) (e.g. GMLC)requests some location service (e.g. positioning) for a target UE to theserving MME.

1c. Or: the serving MME for a target UE determines the need for somelocation service (e.g. to locate the UE for an emergency call).

2. The MME transfers the location service request to an E-SMLC.

3a. The E-SMLC instigates location procedures with the serving eNode Bfor the UE—e.g. to obtain positioning measurements or assistance data.

3b. In addition to step 3a or instead of step 3a, for downlinkpositioning the E-SMLC instigates location procedures with the UE—e.g.to obtain a location estimate or positioning measurements or to transferlocation assistance data to the UE.

3c. For uplink positioning (e.g., UTDOA), in addition to performing step3a, the E-SMLC instigates location procedures with multiple LMUs for thetarget UE—e.g. to obtain positioning measurements.

4. The E-SMLC provides a location service response to the MME andincludes any needed results—e.g. success or failure indication and, ifrequested and obtained, a location estimate for the UE.

5a. If step 1a was performed, the MME returns a location serviceresponse to the UE and includes any needed results—e.g. a locationestimate for the UE.

5b. If step 1b was performed, the MME returns a location serviceresponse to the EPC entity in step 1b and includes any neededresults—e.g. a location estimate for the UE.

5c. If step 1c occurred, the MME uses the location service responsereceived in step 4 to assist the service that triggered this in step 1c(e.g. may provide a location estimate associated with an emergency callto a GMLC).

As a part of the Location service response, the UEs location estimatesare provided by the network.

This known architecture is not suitable for use with non-terrestrialnetworks for direct connectivity because:

-   -   1. The UE can be programmed only manually to wake up at specific        times or wake up randomly to transmit. This is inefficient in        terms of device power management for non-terrestrial use-case.    -   2. The current synchronization mechanism in the protocol cannot        correct more than 7.5 KHz of Doppler inherently.    -   3. There is no transmission of location information of the        base-station which would be necessary to predict the UE wake up        time and possibly high Doppler estimation and correction.    -   4. The current protocol can handle timing offsets of less than 1        ms only (up to 266.67 μS for NB-IoT and 666.67 μS for LTE) which        is equivalent to the cyclic prefix length of the OFDM and SCFDMA        symbols used in the standard. This would be higher for        non-terrestrial networks, especially for satellites.    -   5. For the cellular based location service, the UE needs to be        in view of at least 3 base-stations, which may or may not be the        scenario for non-terrestrial networks.        Proposed Method and Architecture

FIG. 1 illustrates the main steps of a method in accordance with apreferred embodiment of the invention. In a non-terrestrial cellulardata communication network 100, at least one airborne or space bornebase station moves along a flight trajectory at a given altitude h aboveground, wherein h may be in the range of a few to a few hundredkilometers. The method comprises the following main steps:

a) providing flight trajectory data 122′ for said base station 120, i.e.data describing the flight trajectory 122 of the base station 120, in amemory element of a ground based terminal device 110;

b) providing terminal location data 112 in a memory element of saidterminal device 110; the terminal location data comprises an indicationof the terminal device's position on the ground, i.e. in absolutecoordinates. This indication may be obtained by any suitablegeo-positioning means, including but not limited to a satellite basedgeo-positioning system.c) using a data processing unit, determining at least one time slotduring which a wireless communication channel 130 between said terminaldevice 110 and said base station 120 is estimated to be available, basedon said flight trajectory data 122 and on said terminal location data112; Based on the flight trajectory data 122, the processing unit mayfor example extrapolate the data so as to compute estimated futurepositions, velocities etc. of the base station. Further, a channel maybe estimated to be available if the elevation of the base station 120,as observable from the terminal device 110, is above a given andpredetermined threshold value.d) using a data processing unit, scheduling a data reception ortransmission between said terminal device and said base station duringthe determined time slot.

The method corresponds to a non-terrestrial cellular network cognitivemethod for waking-up the terminal device, which thereby reduces itspower consumption. Further details of several embodiments in accordancewith the invention are described here below, without limiting theinvention thereto.

A non-terrestrial cellular system 100 may be understood to include aglobal or local constellation or group of non-terrestrial flying objects120 such as airplanes, drones, HAPS and/or satellites that may belocated at a distance of a few km above the earth's surface up to LowEarth Orbit, LEO, Medium Earth Orbit, MEO, or Geostationary Orbit, GEO.Each of these flying objects ideally hosts a regenerative payload,capable of running a complete base-station+Core network solution, andother allied subsystems. The entire system of a non-terrestrial cellularnetwork composes for example of a multitude of such non-terrestrialpayloads, earth station gateways, GW, non-terrestrial radio accessnetwork, NT-RAN, data centers, non-terrestrial mobile network operators,SMNO, and non-terrestrial mobile or fixed user equipment(s), UE.

Each non-terrestrial object or base station 120 in the fleet orconstellation flies a payload (along with other necessary sub-systemssuch as the Telemetry, Tracking & Control subsystem, power subsystem,thermal control subsystem, Attitude determination and Control subsystem,on-board computer, etc.) that is a complete network in a box solutionbased on software defined radio architecture, SDR.

Each of these payloads may comprise a Base Transceiver System, possiblywith additional Core network functionality (such as MME, S-GW, AUTH,etc., as named in the 3GPP 4G LTE/NB-IoT standard or its equivalent to2G, 3G or 5G standards of 3GPP) depending on the architecture. Thepayload can run all the layers (PHY, MAC, RLC, PDCP, RRC/IP) of the 3GPPcellular protocol stack with and without modifications for bothnon-terrestrial and terrestrial use cases. These aspects are known inthe art as such and their function will not be described to any detailin the context of this description. The description focuses on theconcepts that are most relevant for understanding the present invention.

In cellular 4G LTE/NB-IoT, for example, an eNB is a controller of agroup of cells. A typical cell site will have a single eNB controlling 3sectors (or cells) with a single 51 link back to the Evolved PacketCore, EPC. This is a convenient level of abstraction as it allows asingle centralized processing system coupled with many remote radioheads, RRH, to implement the sectors. If all the scheduling is performedin one location, then this enables spontaneous interference mitigationsschemes without having to exchange information with distant nodes (andthe inherent delays involved in backhauling these messages).

The non-terrestrial cellular system 100 extends this principle in that asingle base-station 129 controls many cells and these are implemented as‘softly’ as possible to allow flexibility and scalability. In 3GPPLTE/NB-IoT, the maximum number of cells that an eNB can control is 256,due to the cell id being an 8-bit quantity. It is assumed that this willbe sufficient for the non-terrestrial cellular application.

The system architecture in which the methods in accordance with aspectsof the invention are put to use may be implemented in one of the threeways as shown in FIGS. 8-10 .

FIG. 8 is the Store-and-forward architecture of a non-terrestrialcellular network 200. If the link with the Gateway is not permanent andthe link between the base-station 220 and the core network (S1 interfaceas it is called in the 3GPP 4G standard or its equivalent for otherstandards such as 2G, 3G or 5G) is not always available, then some ofthe core network functionalities must be incorporated into eachnon-terrestrial payload 220 to compensate for the lack of connectivitywith the gateway. Here, elements such as the Packet Data Network Gateway(PDN-GW; Serving-GW/Packet-GW as called in the 4G standard) andauthentication centre must be localized in the non-terrestrial payloadand updated via some Operation and Maintenance, OAM, procedure.Similarly, any user data must be queued in the non-terrestrial basestation 220 and exchanged with the Gateway when the ground link isavailable. This is the store and forward architecture.

Each non-terrestrial payload effectively acts as a base-station 220 andcore network component and the system is scalable by adding newpayloads, just like a terrestrial network would add a new base station.Scalability is also achievable by increasing the available bandwidth foroperation.

The link with the Gateway is proprietary (could be with the TT&C Linkfor the satellites or equivalent technology for other platforms) andmust provide the following functionality: the link must beBi-directional. It can be off the shelf such as a point to pointmicrowave link. The transport layer can be based on any reliabletransport technology, such as IP. A simple protocol to transfer storeduser datagrams between the non-terrestrial payload and the underlyingcore network connection (presumed to be IP). An OAM (Operations,Administrations, Management) protocol for maintenance of the softwareentities in the non-terrestrial context. One example of this is theadd/modify/delete subscriber records in the Home Subscriber Service,HSS, so that authentication and admission of users can be performed evenwhen the link with core network is unavailable.

In this case, the additional inter-payload links (X2 interface as calledin the 4G LTE/NB-IoT standard) shall enable the interface betweenmultiple non-terrestrial base-stations to achieve real time operations.For example, this could be via inter-satellite links for payloads hostedon satellites.

FIG. 9 is the alternative architecture of a non-terrestrial cellularnetwork 300 with permanent S1 interface. If S1 is always available andreliable, then the IP gateway and the core network are located on theground and may be shared between all non-terrestrial base-stations 320.This is the preferred architecture for higher scalability as eachnon-terrestrial payload acts just as base-station and there is a centralcore network. The link with the Gateway is standardized and may beimplemented in terms of any off the shelf point to point link over IP.However, this requires high complexity as inter-satellite links andgreater ground station coverage to guarantee continuous visibility arerequired.

The X2 interface is an optional 3GPP 4G interface that allows eNBs 320to communicate directly with each other in near real-time. It is used inLTE to enable features such as data forwarding of RRC contexts duringhandover, interference coordination and load balancing and is includedhere for completeness. If S1 is a permanent interface, then X2 will beavailable too as is can use the same transport layer. In cellular NB-IoTX2 is also used to transfer RRC Contexts between eNBs in the user planeoptimization scheme. In terrestrial networks, all base stations arestationary. This means, the X2 interface of a base-station is definedwith respect to its fixed neighbour base-stations for informationexchange between base-stations within the same core network.

However, in a non-terrestrial cellular data network, the base-stationsare moving and dynamic in their trajectories. From a constellationperspective, this means the set of neighbours of a base-station is notfixed, but changing. Therefore, each base-station preferably uses itsown location and trajectory information, and the information of locationand trajectory of the other base-stations in the constellation/fleet(which may be, upon expiry, constantly refreshed by the core-network) toconstantly update its neighbours and the respective definitions of theX2 interface with these updated neighbouring base-stations.

In case of terrestrial networks, this X2 interface is established bymicrowave, optical or other reliable links. In non-terrestrial network,NTN, this may be established through the inter-base station link, incase of satellites it is the inter-satellite link.

FIG. 10 is an alternative femto-cell architecture for a non-terrestrialcellular communication network 400. It has been proposed that instead ofusing a cellular link directly as the link between non-terrestrialplatform 420 and UEs, then a network of ground-based cellularaggregators/gateways (as called femtocells in 4G standards) are deployedand they are backhauled via non-terrestrial payloads instead.

Femtocells can be deployed within buildings for in-building coverage.Femtocells can act as concentrators for groups of cellular terminals,instead of trying to connect each terminal directly to a non-terrestrialconstellation. The shorter radio links between terminals and femtocellswill enable better coverage (or deployment in more hostile radioenvironments). Femtocells feature extensive radio resource managementalgorithms for ad-hoc deployments.

If the frequency available for non-terrestrial backhaul is still used,then the femtocell antenna 410 will still need to be in Line of Sight,LOS, with the non-terrestrial payload 420 which implies the need forfeeders and custom installation. The femtocell could operate onfrequencies more suitable for in-building coverage. If licensed bandsare used, then clearly an operating license will be required, and thiswill vary between territories.

To backhaul the core network interface from the femtocells to thenon-terrestrial payload a new real-time, high availability radio accessscheme is required for this link. A 4G/5G relay could be an option butis subject to similar technical restrictions in the physical layer.

Femtocells will need a permanent power supply, as the power amplifierwill need to be permanently operating to generate the downlink controlchannels for UE synchronization. The technology can be implemented inboth licensed and unlicensed frequency bands.

In order to accommodate non-IP data delivery for store and forwardarchitecture FIGS. 8-10 , Service Capability Exposure Function, SCEF,components shall be a part of the EPC of the non-terrestrial cellularsystem. This connection between the SCEF and the application servers is,on a terrestrial cellular network, a permanent connection.

It is proposed that the SCEF (Service Capability Exposure Function)functions be modified to accommodate for, but not limited to thefunctions of, buffering of non-IP user data in SCEF from multiple users,mimicking the connection between SCEF and Application Servers on theground station in the absence of an intersatellite (inter base-station)link. In the presence of an available link the link between the SCEF andthe application servers may be made permanent and implemented asrecommended by the 3GPP cellular standard.

The non-terrestrial airborne or spaceborne base-station 120, 220, 320,420 may have single or multiple spot beams in the uplink and downlink todivide the radio and processing overhead spatially and provide methodsof frequency reuse. The actual design of the non-terrestrial beam shouldtake into consideration to be as flexible as possible in the MAC and PHYdesign. The beams can cover same cell area or different cell areas. Thisis valid for both Frequency Division Duplexing, FDD, and Time DivisionDuplexing, TDD, operations of the non-terrestrial cellular system. Thesystem may have single-carrier or multi-carrier capabilities and usedifferent modulation techniques such as GMSK, QPSK, QAM, etc., or anyequivalent variants of these schemes. The access scheme may be TDMA,CDMA, FDMA, OFDM, OFDMA etc., or any other equivalent schemes. Thehigher layers of the protocol may or may not be agnostic to theduplexing scheme employed in the payload and UE operation of thenon-terrestrial cellular system.

Transmission of Flight Trajectory Data

One novel addition to such a system to enable non-terrestrial deploymentwould be the introduction of the continuous or periodic transmission ofan enriched data-set by the airborne or spaceborne base-station 120,220, 320, 420. For example, the Two Line Element, TLE, for satellites orequivalent for other non-terrestrial base-station embodiments) relatedto its path information on the downlink of the cellular protocol as apart of its broadcast of system information.

This data set 122′ may be used by the respective terminal devices 110,210, 310, 410 such as UEs/gateways in combination with their knowledgeof their own location 112 to predict the state of the base-station 110,210, 310, 410 such as its position and possibly velocity, Doppler anddelay, etc., using a suitable propagator algorithm (such as the SGP4available at help.agi.com at the following path:/stk/index.htm#stk/vehSat_orbitProp_msgp4.htm propagator for satellitesand equivalent propagators for other embodiments of non-terrestrialbase-stations) or a definite period in the future with a valid accuracy.This computed information may further be used by the UEs/Gateways 110,210, 310, 410 to wake-up, synchronize, perform Doppler and delaypre-compensation and attach themselves to the network 100, 200, 300,400.

This information of the UE location and the base station location isused by the propagator (in this example an SGP4 propagator, withoutlimiting the invention thereto) to estimate the UE wake-up times andDoppler and delay prediction at the application and management layer.The input of SGP4 algorithm, shown in FIG. 11 , is a two-line elements(TLE) produced by NORAD. The TLE is a description of the current orbitalelements of a satellite. From the TLE, the SGP4 algorithm can calculatethe inertial orbital state vectors of a satellite at any point T in thefuture (or the past) to some accuracy. The raw outputs of the SGP4 arethe position and the velocity vector of the satellite in an inertialreference frame (ECI) called True equator, Mean equinox (TEME). The TLEin this example is a data format encoding the mean orbital parameters ofa satellite for a given point of time. Here is an example in itsstandard format:

1 43132U 18004X 19215.41404522 0.00000829 00000-0 37553-4 0 9992

2 43132 97.4860 283.1384 0011471 94.2701 265.9847 15.23639096 86464

The first line includes the following elements from left to right:

-   -   Line number    -   Satellite number    -   International designator    -   Epoch year & Julian day fraction    -   1^(st) derivative of mean motion or ballistic coefficient    -   2^(nd) derivative of mean motion, usually blank    -   Drag term or radiation pressure coefficient    -   Ephemeris type    -   Element number & check sum

The second line include the following elements from left to right:

-   -   Line number    -   Satellite number    -   Inclination    -   Right ascension of the ascending node    -   Eccentricity    -   Argument of perigee    -   Mean anomaly    -   Mean motion    -   Revolution number & check sum

This standard format can be customized to the specific needs of theapplication, without departing from the scope of the present invention.

By knowing the position and velocity of the base station and the userterminal, it is possible to calculate when the user terminal needs towake up but also the Doppler shift and the propagation delay at anypoint T in the future to some accuracy.

A way to predict the wake-up times is to calculate the elevation of thebase stations above each user terminal. The user terminal shall wake upwhen the elevation of a base station is higher than a predefinedthreshold (min_elev in the flowchart shown in FIG. 12 ).

A way to decrease the resources needed to compute the wake-up table isto use a dynamic step instead of a fixed one. Several techniques can beused like increasing the step size after the elevation of the satelliteis higher than the minimum elevation required to have a successfulconnectivity. The step size can also be increased or decreased infunction of the distance between the base station and the user terminal.The step size can also be changed in function of the derivative in timeof the distance between the base station and the user terminal, as longas the base station is going away of the user terminal, the step sizecan be big while when the distance is reducing and closer than athreshold, the step size should be small to avoid to miss a base stationin the field of view.

The dataset 122′ of each base-station 120 may preferably have an expiryperiod. For example, the TLE includes mean orbital elements and tocompensate varying over time non-conservative forces that impact thesatellite orbit like the atmospheric drag or the solar radiation, itmust be updated regularly. This means that the dataset 122′ must beupdated on a regular basis for the UE 110 to be able to make accuratepredictions. The TLE updates can be transmitted by each base station (incase of a satellite constellation, by each satellite) as a part of thebroadcast and system information continuously or periodically (forexample, in case of LTE/NB-IoT, the System Information Block-16 (SIB16)which is an RRC message.) This is received by the UE from all thenon-terrestrial base stations 120 and regularly updated before theexpiry of the current dataset. Alternatively, if a fleet orconstellation of airborne/spaceborne base station is used, one basestation may transmit, preferably via broadcast, a dataset 122′ relatingto a plurality of base stations, preferably of all base stations, withinthe fleet or constellation.

The UE wake-up times are either scheduled by application and managementlayer either for scheduled transmission or to update its data-setalmanac in a storage or memory element.

For the cellular protocol to work over a non-terrestrial scenario, theknown protocol sequence as shown in FIG. 2 is therefore modifiedin-order to transmit the base-station location information (for examplesatellite TLE information via the SIB 16 of the NB-IoT/LTE protocol)which shall be used by the UE for wake up and advantageously also foroffset prediction and pre-compensation as shown in FIG. 13 .

At the terminal device 110 (UE, GW) level, the following functionalitiesare added to the existing architecture as shown in FIG. 14 :

-   -   1. Reception of base-station location information (for example,        TLE for satellites or equivalent for HAPS, drones and airplanes)    -   2. A propagator to predict to predict future visibilities of the        non-terrestrial base-station for UE wake up, Doppler and delay        prediction    -   3. feedback of predicted Doppler and delay to pre-compensate UE        transmissions    -   4. Preemptive scheduling at the MAC layer to compensate the        fixed round trip delay

None of the currently known cellular network technologies provide thisfeature, as it was not necessary for terrestrial deployments. This isbecause in terrestrial deployments the base-stations are fixed andimmovable as envisaged by the state of art. But for non-terrestrialnetworks the inclusion of this information as a part of its downlinktransmission, enable the UEs/gateways to optimize their power savingmechanisms by scheduling their wake-up procedures, optimize theirsynchronization, and enable Doppler and delay (caused by the highvelocity and high altitude scenarios of the non-terrestrial networkdeployment scenarios) estimation, correction and pre-compensations fortheir corresponding uplink transmissions.

Estimation of the Location of a Terminal Device

A ground based terminal device 110 or UE may know its location due toits fixed nature, or from GPS/GNSS or an equivalent positioning system,or it is able to triangulate itself. If the UE does not know its ownlocation, it shall be able to triangulate itself using cellular basedlocation services such as the use of downlink observed time differenceof arrival, OTDOA, or uplink time difference of arrival, UTDOA, Enhancedcell-ID as mentioned in the 3GPP cellular standards or other equivalenttechniques assisted by multiple non-terrestrial base-stations over asingle pass or over multiple passes.

If the UE's location is unknown and does not have a positioning deviceas a part of its architecture, it may initiate a cellular based locationprocedure. While other positioning algorithms may be used within thecontext of the present invention, the following procedure is provided byway of example. It allows the network to triangulate the UE's 110location and report back to the UE a location estimate 112. For this,the UE first performs a random wake-up and blind acquisition of thecarrier that is made available by the airborne/spaceborne cellular basestation, and uses offset estimates of the most recent downlink topre-compensate its uplink transmissions to attach itself to the network.Once it attaches successfully to the network via one of thebase-stations, it shall initiate the location procedure. This may bedone instantaneously if there are at least 4 non-terrestrial basestations simultaneously visible to the UE or over multiple passes if atleast 2 such non-terrestrial base stations are visible to the UE at anygiven time.

If the UE 110 does not have knowledge of its own location (FIG. 16 ,step 1000, where NTN denotes the non-terrestrial network, i.e.airborne/spaceborne base station) by its fixed nature or through apositioning device such as GPS/GNSS, it shall wake up at random times toinitially acquire a carrier that is made available by theairborne/spaceborne base station, and synchronize itself to the network.This may be done by standard synchronization procedures of the 3GPPstandard, for example. However, if the actual frequency error betweenthe base-station and the UE can be larger than the prescribed detectionwindow then the UE must perform several passes at detection, either byre-tuning its reference oscillator according to some raster scheme or bypre-rotating received samples to simulate a change in downlink carrierfrequency. Hence, by taking multiple passes and frequency-binning theresults the feasible downlink frequency space can be searchedeffectively. For example, with a LEO satellite constellation for NB-IoTservice, assuming a hypothetical maximum Doppler error of 135 KHz isencountered when the satellites are at a low elevation. In this case, byway of example:

-   -   The UE should perform many frequency-binned searches according        to some raster scheme. Even if the NPSS/NSSS detection window is        as large as 5 kHz, then this is 135/5=27 searches on a 5 kHz        raster to cover just one potential downlink frequency from a        satellite. If the UE has no prior knowledge of which frequencies        are in use, then the entire downlink frequency band must be        scanned exhaustively to acquire the system. This will very        lengthy and cause a severe drain on resources for battery        powered devices.    -   The maximum Doppler frequency error of 135 kHz is ambiguous on        the EUTRA carrier frequency allocation scheme, which uses a 100        kHz raster. If NPSS/NSSS are detected do they belong to carrier        N, N−1 or N+1 in the presence of worst-case Doppler error?        Clearly, this can be resolved by the UE observing the frequency        change in NPSS/NSS over a predetermined period to see if it        converges (as the satellite approaches the zenith) or by        encoding the actual downlink frequency EARFCN somewhere in the        system information, but this is a non-standard extension that        must be added to the UEs search procedure such as modifying the        raster window.

For example, the channel raster in standard terrestrial NB-IoT is 100KHz. This means that the receiver's search window can only scan anddetect 100 KHz bandwidth at a time and then it moves to the next searchwindow of 100 KHz if it doesn't find any operating carrier frequency.First, consider the elevation angle of 70°. In order to capture allDoppler shifts, we propose a reduced channel raster of less than 100Khz, and specifically for example 50 KHz at 70° elevation angle, withoutlimiting the invention to this raster size. Using 70 KHz detectionwindow and 50 KHz raster search will be able to detect carriers shiftedby any Dopplers. The working of 50 KHz raster is illustrated in FIGS.17A and 17B. The figures illustrate the expected frequency FO, asindicated for example by a synchronization signal received from the basestation, and the actual frequency Fd, at which the signal was indeedreceived, caused by a Doppler shift. FIG. 17A shows the 1st rastersearch in which a Doppler of 45 KHz for example is missed because itfalls outside the range of detection window (which is +/−35 KHz). In the2nd search, illustrated by FIG. 17B, the channel raster is shifted by 50KHz instead of 100 KHz. Hence, the carrier is detected here. In the sameway, higher Doppler shifts can also be detected with subsequentsearches.

Once this is successful, the UE/gateway 110 may request a locationservice from the network via the airborne/spaceborne base-station 120that is within this terminal device's line of sight. Once the network100, via its core services, provides back the assistance data, theUE/Gateway can perform, and report back the RSTD measurements. Thisshall be used by the network to provide back a location estimate 112 tothe UE/gateway.

With reference to FIG. 16 , the proposed method may therefore besummarized as follows. At step 1100, the terminal device performs ablind acquisition of the signal carrier provided by one of the basestations. If it synchronizes with one base station (1100), it may thenat step 1112 download only limited synchronization assistanceinformation from that base station. This is iterated to accumulatesynchronization assistance information over time. If at step 1100 itsynchronizes to a pair of base stations, it then acquires its locationthrough successive cellular triangulation and downloads synchronizationassistance information from the base stations at step 1122. Then theterminal device may use pre-determined wake-up slots, location andsynchronization information, as well as Doppler and delay compensationin accordance with aspects of the invention at step 1124. If theterminal device loses the channel provided by the bases station due to alocation change at step 1126, it goes back to step 1100. If after step1100 the base station succeeds with synchronizing with multiple basestations at step at step 1130, it proceeds to step 1132 according towhich it may acquire its location through direct cellular triangulation,and it may download base station synchronization assistance information.

It is to be noted that now, for a non-terrestrial network, it may or maynot be possible for the UE to have view of 4 airborne or spaceborne basestations at all times. Therefore, a modified measurement procedure isproposed. In this case, only one or two base-stations are required to bein view of the terminal device, be it a user equipment or a gateway. TheUE/Gateway shall initiate a location service request when its locationis unknown to one of the visible airborne/spaceborne base-stations,which shall be used as the reference base station. This request isforwarded to the location server in the network through the inter-basestation link. The core network will direct all the base stations thatare expected to be visible to the UE to prepare preemptively thelocation assistance data to be transmitted to the UE when they arevisible, this is because the core network knows the paths, trajectoriesand instantaneous location of each airborne/spaceborne base stationwithin the non-terrestrial cellular network.

If only one airborne or spaceborne base station is available to theterminal device at any time, the fact that the base station's positionwith respect to the terminal device constantly evolves along the basestation's trajectory may preferably be used to accumulate multiplemeasurements of the positioning reference signal, PRS, having beentransmitted from said same base station, but at different positionsalong its trajectory, and at correspondingly different time instants.

If a pair of base stations is visible to the terminal device, the lattermakes a first set of measurements of the positioning reference signals,PRS, for the pair of visible base-stations and stores the result in amemory element. Once the next two base-stations are in view, at a latertime, the UE makes the second set of measurements and so on. In such afashion the UE/gateway may accumulate multiple sets of reference signaltime difference, RSTD, measurement for the purpose of 3-dimensionalaccuracy (at least 3 sets of RSTD measurements) over consecutive passesand report them back to the core network.

Similarly, if 3 or more base-stations are visible at the same time thesemeasurements can be performed simultaneously and in real time andreported back to the core network. The core network now uses theseinformation sets to compute and provide back a location estimate to theUE via the next visible base station to the UE/gateway. The locationserver at the core network will assume the original locations of thebase-stations at the time when the respective PRS signals were sent,even if the location has changed due to the continued motion of thebase-stations.

The UE/gateway 110 may use this location estimate 112 as its ownlocation in combination with the received TLE 122′ of the satellites andupdate its wake-up schedules, further carrier search, synchronizationprocedures, Doppler and delay estimation and pre-compensations.

Time and Frequency Synchronization

System operation, in a non-terrestrial scenario, i.e. the communicationbetween a ground based terminal device 110 (user equipment or gateway)and an airborne/spaceborne cellular base station 120, has to deal withfrequency and time errors in order to acquire, synchronize and establishconnectivity.

-   -   The frequency error f_(err) in the system can be caused by        primarily:    -   1. Doppler shift f_(d) over the pass due to non-terrestrial        movement of the base-station 120    -   2. Crystal offset error of the local oscillator f_(LO)    -   3. Drift in the Doppler shift due to the scheduling latencies

In the context of the present description, the Doppler shift and theDoppler drift (derivative of the Doppler shift in time) arecharacterized by their normalized values. Using normalized values keepsthe values independent of the carrier frequency used. The normalizedDoppler shift is the Doppler shift divided by its carrier frequency. Thenormalized Doppler drift is the Doppler drift divided by its carrierfrequency.

Assuming for example an overhead pass of a satellite base-station 120 inthe LEO orbit at 600 Km 60° to 60° elevation over a ground-basedterminal device 110, Table I shows the variation of Doppler shift andpropagation delay with respect to the elevation angle. The system wouldtherefore experience a maximum normalized Doppler of up to 1.17e-05 anda maximum normalized Doppler drift of up to 2.00e-03.

TABLE 1 Doppler and propagation delay Normalized Normalized Time DopplerDoppler Propagation Prop delay RTD Elevation [sec] shift drift[s^(−l)]delay [sec] drift [sec] 60 0.00 1.17E−05 −2.02E−07 2.28E−03 −1.16E−054.56E−03 60.5 0.87 1.15E−05 −2.05E−07 2.27E−03 −1.14E−05 4.54E−03 611.74 1.13E−05 −2.08E−07 2.26E−03 −1.12E−05 4.52E−03 61.5 2.60 1.11E−05−2.11E−07 2.25E−03 −1.10E−05 4.50E−03 62 3.45 1.09E−05 −2.13E−072.24E−03 −1.09E−05 4.48E−03 62.5 4.29 1.08E−05 −2.16E−07 2.23E−03−1.07E−05 4.46E−03 63 5.13 1.06E−05 −2.18E−07 2.22E−03 −1.05E−054.44E−03 63.5 5.96 1.04E−05 −2.21E−07 2.21E−03 −1.03E−05 4.43E−03 646.79 1.02E−05 −2.24E−07 2.20E−03 −1.01E−05 4.41E−03 64.5 7.61 1.00E−05−2.26E−07 2.20E−03 −9.95E−06 4.39E−03 65 8.42 9.85E−06 −2.29E−072.19E−03 −9.76E−06 4.38E−03 65.5 9.23 9.67E−06 −2.31E−07 2.18E−03−9.58E−06 4.36E−03 66 10.04 9.48E−06 −2.33E−07 2.17E−03 −9.39E−064.34E−03 66.5 10.83 9.30E−06 −2.36E−07 2.16E−03 −9.20E−06 4.33E−03 6711.63 9.11E−06 −2.38E−07 2.16E−03 −9.02E−06 4.32E−03 67.5 12.41 8.92E−06−2.41E−07 2.15E−03 −8.83E−06 4.30E−03 68 13.20 8.74E−06 −2.43E−072.14E−03 −8.64E−06 4.29E−03 68.5 13.97 8.55E−06 −2.45E−07 2.14E−03−8.45E−06 4.27E−03 69 14.75 8.36E−06 −2.47E−07 2.13E−03 −8.26E−064.26E−03 69.5 15.52 8.17E−06 −2.50E−07 2.12E−03 −8.07E−06 4.25E−03 7016.28 7.98E−06 −2.52E−07 2.12E−03 −7.88E−06 4.24E−03 70.5 17.04 7.78E−06−2.54E−07 2.11E−03 −7.69E−06 4.22E−03 71 17.80 7.59E−06 −2.56E−072.11E−03 −7.50E−06 4.21E−03 71.5 18.55 7.40E−06 −2.58E−07 2.10E−03−7.30E−06 4.20E−03 72 19.30 7.21E−06 −2.60E−07 2.09E−03 −7.11E−064.19E−03 72.5 20.05 7.01E−06 −2.62E−07 2.09E−03 −6.92E−06 4.18E−03 7320.79 6.82E−06 −2.64E−07 2.08E−03 −6.72E−06 4.17E−03 73.5 21.53 6.62E−06−2.66E−07 2.08E−03 −6.53E−06 4.16E−03 74 22.26 6.43E−06 −2.68E−072.07E−03 −6.33E−06 4.15E−03 74.5 22.99 6.23E−06 −2.69E−07 2.07E−03−6.13E−06 4.14E−03 75 23.72 6.04E−06 −2.71E−07 2.07E−03 −5.94E−064.13E−03 75.5 24.45 5.84E−06 −2.73E−07 2.06E−03 −5.74E−06 4.12E−03 7625.17 5.64E−06 −2.74E−07 2.06E−03 −5.54E−06 4.11E−03 76.5 25.89 5.44E−06−2.76E−07 2.05E−03 −5.34E−06 4.11E−03 77 26.61 5.25E−06 −2.78E−072.05E−03 −5.15E−06 4.10E−03 77.5 27.32 5.05E−06 −2.79E−07 2.05E−03−4.95E−06 4.09E−03 78 28.04 4.85E−06 −2.80E−07 2.04E−03 −4.75E−064.08E−03 78.5 28.75 4.65E−06 −2.82E−07 2.04E−03 −4.55E−06 4.08E−03 7929.45 4.45E−06 −2.83E−07 2.04E−03 −4.35E−06 4.07E−03 79.5 30.16 4.25E−06−2.84E−07 2.03E−03 −4.15E−06 4.06E−03 80 30.86 4.05E−06 −2.86E−072.03E−03 −3.95E−06 4.06E−03 80.5 31.57 3.85E−06 −2.87E−07 2.03E−03−3.75E−06 4.05E−03 81 32.27 3.65E−06 −2.88E−07 2.02E−03 −3.55E−064.05E−03 81.5 32.97 3.45E−06 −2.89E−07 2.02E−03 −3.35E−06 4.04E−03 8233.66 3.25E−06 −2.90E−07 2.02E−03 −3.14E−06 4.04E−03 82.5 34.36 3.04E−06−2.91E−07 2.02E−03 −2.94E−06 4.03E−03 83 35.05 2.84E−06 −2.92E−072.02E−03 −2.74E−06 4.03E−03 83.5 35.75 2.64E−06 −2.92E−07 2.01E−03−2.54E−06 4.03E−03 84 36.44 2.44E−06 −2.93E−07 2.01E−03 −2.34E−064.02E−03 84.5 37.13 2.23E−06 −2.94E−07 2.01E−03 −2.13E−06 4.02E−03 8537.82 2.03E−06 −2.94E−07 2.01E−03 −1.93E−06 4.02E−03 85.5 38.51 1.83E−06−2.95E−07 2.01E−03 −1.73E−06 4.01E−03 86 39.19 1.63E−06 −2.96E−072.01E−03 −1.53E−06 4.01E−03 86.5 39.88 1.42E−06 −2.96E−07 2.00E−03−1.32E−06 4.01E−03 87 40.57 1.22E−06 −2.96E−07 2.00E−03 −1.12E−064.01E−03 87.5 41.25 1.02E−06 −2.97E−07 2.00E−03 −9.15E−07 4.01E−03 8841.94 8.14E−07 −2.97E−07 2.00E−03 −7.12E−07 4.00E−03 88.5 42.62 6.10E−07−2.97E−07 2.00E−03 −5.09E−07 4.00E−03 89 43.31 4.07E−07 −2.97E−072.00E−03 −3.05E−07 4.00E−03 89.5 43.99 2.03E−07 −2.97E−07 2.00E−03−1.02E−07 4.00E−03 90 44.68 0.00E+00 — 2.00E−03 — 4.00E−03

The timing error t_(err) in the system can be caused by primarily:

-   -   1. Propagation delay t_(d) over the pass due to non-terrestrial        movement of the base-station 120    -   2. Local oscillator clock error t_(LO)    -   3. Variation of the propagation delay caused by change in        base-station position during the pass due to scheduling        latencies

Table depicts the variation of round-trip delay (RTD) between 30°elevation to 90° elevation for the same example.

TABLE 2 Elevation angle vs Round Trip Delay Elevation angle Round TripDelay (RTD) [ms] 30° 7.17 35° 6.45 40° 5.89 45° 5.44 50° 5.08 55° 4.7960° 4.56 65° 4.38 70° 4.24 75° 4.13 80° 4.06 85° 4.02 90° 4.00

It is to be noted that that for a 60° to 90° elevation pass, all the UEsexperience a constant integral part of the RTD is 4 ms. The fractionalpart varies between 0.001 ms to 0.560 ms.

In accordance with a preferred embodiment of the invention, the overallsystem operation may be broken down into 3 functional steps, namely; UEwake-up, frequency and time synchronization and protocol operation. Inorder to establish connectivity, the UE must be able to wake up duringan available satellite pass, acquire the carrier frequency estimate andtrack the relative shift of frequency and delay pre-compensated by theUE before every uplink transmission.

The first step is for the ability of the UE to determine when it mustwake-up in order to establish communication with the base-station. Forthis purpose, the UE must have the enriched dataset regarding the pathinformation of each base-station (in this example the satellite TLE) andits own geo-location. The UE operation when its own location is known isshown in FIG. 18 , starting at step 2000. At step 2100, the terminaldevice/UE performs a blind acquisition procedure of a carrier channel.At step 2110, it synchronizes with one airborne/spaceborne base station,and downloads only limited base station synchronization information atsubsequent step 2112, before iterating the process so as to accumulatesynchronization information over time. Once enough assistanceinformation has been accumulated, the terminal device computes at step2120 predicted values or estimates for the base station's position,experiences Doppler offset/drift, timing delay, etc. . . . in accordancewith aspects of the invention. This information is then used at step2122 to wake-up in accordance with the so-computed wake-up slots, and tocompensate for Doppler and delay errors on the carrier. Once thetrajectory information for a given base station expires at step 2124,the acquisition loop is started again at step 2110.

Using this concept, as an example, let us consider the proceduresinvolved for NB-IoT. The first step to synchronization is to acquire theframe and the symbol timing and the frequency from the synchronizationsignals at the physical layer. This is done by the timing and frequencysynchronization block in the UE architecture as shown in FIG. 14 . Thisblock estimates the Doppler and offset of the frequency. For frequencysynchronization, the process involves:

-   -   Frequency estimation: This is the process of estimating the        complex frequency components of a signal in the presence of        noise or channel impairments.    -   Frequency compensation: Once the frequency is estimated, next        the deviation of the local clock frequency with the estimated        frequency is computed and this computed frequency deviation is        compensated to decode the further channels.    -   Frequency tracking: once the frequency is estimated and        deviation is computed, then it has to be constantly monitored        and tracked to keep the deviation under a certain limit.    -   Pre-compensation: The estimated frequency offset, and timing        error is used as input to drive the analog front end to        pre-compensate the uplink transmission.

The acquired timing and frequency of the downlink allows the UE toinitially synchronize itself relative to the non-terrestrial payloaddecode the broadcast channel and other downlink transmissions.

This estimated frequency offset is also input to the timing andfrequency control block which is used to pre-compensate and correct theDoppler shift and frequency and timing errors of the local oscillatorduring the uplink transmission. To that effect a time compensation valuemay be added to the timing of a scheduled data transmission, and afrequency compensation value may be added to the transmission frequencyof a scheduled data transmission to the base-station. Alternatively,this estimated frequency offset and the predicted mean/median/min/actualDoppler drift is also input to the timing and frequency control blockwhich is used to pre-compensate and correct the Doppler shift, theDoppler drift and frequency and timing errors of the local oscillatorduring the uplink transmission. Indeed, as detailed in Table 1, theDoppler drift is always negative and comprised within a predefinedrange. Pre-compensating the uplink using the estimated frequency offsetand then adding the min/mean/median/actual Doppler drift is going toreduce the residual frequency offset during the uplink transmission.

Alternatively, the timing and frequency synchronization block may beassisted by external inputs from the Doppler and delay prediction mapcomputed by the application and management layer to achievesynchronization and implement pre-compensation. For example, filteringthe measured delay and Doppler using the predicted values can increasethe accuracy.

For example, knowing the position of the user terminal and the basestation allows to determine the Doppler shift and the delay ofpropagation.

-   -   {right arrow over (r₁)} is the Earth Centred Earth Fixed (ECEF)        position of the user terminal {right arrow over (r₂)} is the        ECEF position of the satellite    -   {right arrow over (r₃)} is the relative position between the        user terminal and the satellite={right arrow over (r₂)}−{right        arrow over (r₁)}

It is relatively easy to calculate the Doppler shift and the propagationdelay if all state vectors are in the same frame as ECEF (Earth Centred,Earth Fixed frame). The indices “sat” and “ue” are respectively thesatellite and the user terminal.

${p_{sat} = {\lbrack {x_{sat};y_{sat};z_{sat}} \rbrack = {\overset{arrow}{r_{2}}\lbrack m\rbrack}}}{v_{sat} = {\lbrack {\overset{.}{x_{sat}};\overset{.}{y_{sat}};\overset{.}{z_{sat}}} \rbrack \lbrack {m/s} \rbrack}}{P_{ue} = {\lbrack {x_{ue};y_{ue};z_{ue}} \rbrack = {\overset{arrow}{r_{1}}\lbrack m\rbrack}}}{v_{ue} = \lbrack {0;0;0} \rbrack}{c = {{speed}{of}{an}{electromagnetic}{{wave}\lbrack {m/s} \rbrack}}}$Here the speed of the user terminal in ECEF is considered as zero. Thismeans that the user terminal is not moving.

The propagation delay is simply the distance between the user terminaland the satellite (the range) divided by the velocity of anelectromagnetic wave (c) as shown below.

${delay}_{prop} = {\frac{( {{p_{sat} - p_{ue}}} )}{c} = {\frac{\overset{arrow}{r_{3}}}{c}{\frac{\lbrack m\rbrack}{\lbrack {m/s} \rbrack}\lbrack s\rbrack}}}$

The Doppler shift is the dot product between the velocity vector of thesatellite and the normalized range vector.

${doppler}_{shift} = {{( {\overset{arrow}{V_{sat}}\frac{\overset{arrow}{r_{3}}}{\overset{arrow}{r_{3}}}} )*\frac{frequency}{c}}{{( {\lbrack {m/s} \rbrack*\frac{\lbrack m\rbrack}{\lbrack m\rbrack}} )*\frac{\lbrack{Hz}\rbrack}{\lbrack {m/s} \rbrack}}\lbrack{Hz}\rbrack}}$

It is to be noted that, at the physical layer, only the timing errors ofthe LO are corrected and pre-compensated for the uplink transmission butthe shift in the Time of Arrival, TOA, due to the propagation delay isnot corrected by this block. This is taken care of by the Timing Advancecommand which is issued by the EnodeB as a part of the MAC layer messageto the UE. This is described in the following section.

It is to be noted that as discussed in Table II, UEs experience around-trip delay, RTD, of at least 4 ms which can be always taken intoaccount and ignored by the scheduler (of the eNodeB) for uplinkreception or by means of pre-emptive resource assignment by the UE MACcontroller considering the fixed part of RTD.

For the fractional part of the RTD, at 70° elevation the variation is0.240 ms which can be communicated by the timing advance command fromthe MAC layer of the EnodeB as a part of the Random-Access Responsemessage (for NPRACH) or the MAC Control Element (MAC CE) for the NPUSCH.The timing advance concept is depicted in FIGS. 19A and 19B.

After a UE 110 has first synchronized its receiver to the downlinktransmissions received from the ENodeB 120, the initial timing advanceis set by means of the random-access procedure. This involves the UEtransmitting a random-access preamble on the uplink from which theeNodeB estimates the initial uplink timing offset. The EnodeB respondswith a 11-bit initial timing advance command contained within theRandom-Access Response (RAR) which is a MAC layer message. This timingadvance value is used by the UE to align itself in time for theconsecutive uplink transmission.

To describe the system operation, the example scenario in FIG. 20 isconsidered.

Once the UE 110 wakes up at T₀ and take T₁ ms to acquire the downlinksynchronization signals (NPSS and NSSS) and achieve frame and timesynchronization relative to the satellite EnodeB 120, after which it candecode the broadcast signals at T₂ ms while tracking the downlink(frequency shift and delay shift). It can estimate the frequency anddelay shift on the downlink until T₂ ms. The UE waits for next availablerandom-access window and it transmits the random-access preamble at T₃ms. However, the UE is unable to estimate the shift in the frequency anddelay during this random-access latency period (UE processing time forpreamble preparation and transmission) of T₃−T₂ ms. The UE shalltransmit the random-access preamble at T₃ after pre-compensating thisshift in delay and frequency estimated at T₂. An alternative would be toadd the mean, median, the min or the predicted doppler and time drift tothe delay and frequency estimation as follows:

For the frequency offset using the mean Doppler drift:F_(overall)=F_(DLestim)+F_(mean-drift)*t_(DL-UL latency).

For the frequency offset using the median Doppler drift:F_(overall)=F_(DLestim)+F_(median-drift)*t_(DL-UL latency).

For the frequency offset using the minimum Doppler drift:F_(overall)=F_(DLestim)+F_(min-drift)*t_(DL-UL latency).

Other filtering methods may be used without departing from the scope ofthe present invention.

However, once it has transmitted the random-access preamble, the UEswitches to receive mode and is able to track the downlink again usingthe reference symbols on the downlink and the cyclic prefix of thereceived downlink messages.

The base-station 120 receives the preamble at T₄ ms, estimates the shiftin delay of the preamble caused during T₄−T₂ ms for demodulation of thepreamble and prepares the Random Access Response (RAR) and transmits itwithin the minimum RAR latency period (which is configurable by thescheduler and takes into the account the integer part of the propagationdelay to pre-schedule the RAR) at T₅ ms. The base-station can alsocommunicate the relative shift (fractional part of the RTD) in theactual time of arrival of the preamble against the expected time ofarrival in the Timing Advance Command.

While the UE 110 is listening to the downlink for its corresponding RAR,it is able to keep the synchronization and estimates the shift infrequency and delay at the physical layer until it receives it RAR at T₆ms. This is taken as input by the UE's MAC controller to prepare andpre-compensate the uplink shared channel transmission at T₇ ms. However,the UE is unable to track the frequency and delay shift during theuplink shared channel latency period of T₇−T₆ ms and transmits theuplink shared channel packet without pre-compensating for the shiftsduring this period. An alternative would be to add the mean, median, themin or the predicted Doppler and time drift to the delay and frequencyestimation as follow for the frequency offset using the mean Dopplerdrift F_(overall)=F_(DLestim)+F_(mean-drift)*t_(DL-UL latency). Once itends transmission, the UE can resume tracking the downlink from T₇ msuntil T₁₂ ms.

The base-station receives the uplink shared channel packet with theshift of frequency and delay not compensated by the UE for the uplinkshared channel latency period (UE processing for packet preparation) anddecodes the uplink shared channel. The efficiency of this frequency anddelay shift estimator at the base-station is a deterministic factor onthe tolerance of the error in pre-compensation by the UE for the uplinkdue to the latency periods between the time of reception of the lastdownlink to the corresponding uplink transmission.

This sequence is valid for all consecutive uplink and downlinktransmissions for the entire protocol sequence as long as the estimatorperforms with the acceptable tolerance. Otherwise, if at some limit theestimator's error exceeds the tolerance, a resynchronization required.

The described aspects in accordance with the embodiments may be used asdistinct methods (for UE localization, for Doppler/Time pre-compensationon the uplink, etc. . . . ) or they may be combined with one anotherunless specifically stated otherwise, without departing from the scopeof the invention. It will be understood that the present disclosureincludes all feature combinations. Specifically, the claimed featuresmay be combined.

It should be noted that features described for a specific embodimentdescribed herein may be combined with the features of other embodimentsunless the contrary is explicitly mentioned. Based on the descriptionand figures that has been provided, a person with ordinary skills in theart will be enabled to develop a computer program for implementing thedescribed methods without undue burden.

It should be understood that the detailed description of specificpreferred embodiments is given by way of illustration only, sincevarious changes and modifications within the scope of the invention willbe apparent to the person skilled in the art. The scope of protection isdefined by the following set of claims.

What is claimed is:
 1. A control method for a terminal device in anon-terrestrial cellular data communication network, the networkcomprising at least one airborne or spaceborne base station moving alonga flight trajectory, for connecting the terminal device to said network,the method comprising the following steps: providing flight trajectorydata for said base station in a memory element of said terminal device;providing terminal location data in a memory element of said terminaldevice; using a data processing unit, determining at least one time slotduring which a wireless communication channel between said terminaldevice and said base station is estimated to be available, based on saidflight trajectory data and on said terminal location data; using a dataprocessing unit, scheduling a data reception or transmission betweensaid terminal device and said base station during the determined timeslot; receiving during said scheduled time slot and at said terminaldevice a synchronization signal from said base station, thesynchronization signal carrying data indicating a transmissionfrequency; computing, using a data processing unit of the terminaldevice, an observed Doppler shift based on a receiving frequency forsaid synchronization signal and on the transmission frequency indicatedtherein; and pre-emptively compensating, at the terminal device, asubsequent transmission frequency by a frequency compensation valueduring a subsequent data transmission to said base station, saidfrequency compensation value taking into account said observed Dopplershift.
 2. The method according to claim 1, wherein said terminal devicecomputes, using said data processing unit and based on said flighttrajectory data, estimate values for said base station, comprising anyof a future position, elevation, velocity, Doppler shift, Doppler drift,propagation delay, derivatives of said Doppler shift or propagationdelay, or any combination thereof, and stores these estimate values in amemory element.
 3. The method according to claim 1, wherein saidterminal device computes, using said data processing unit and based onsaid flight trajectory data, estimate values for said base station,comprising any of a future position, elevation, velocity, Doppler shift,Doppler drift, propagation delay, derivatives of said Doppler shift orpropagation delay, or any combination thereof, and stores these estimatevalues in a memory element, and wherein said frequency compensationvalue further takes into account any of said estimated Doppler shiftvalues, Doppler drift, a derivative of the observed Doppler shift, orany combinations thereof, at the time of said subsequent datatransmission.
 4. The method according to claim 1, wherein thesynchronization signal further carrying data indicating timinginformation, wherein the terminal computes, using a data processing unitan observed time shift based on a reception time of said synchronizationsignal and on the timing information indicated therein, and wherein theterminal device pre-emptively compensates the scheduled time oftransmission by a time compensation value during a subsequent datatransmission to said base station, said time compensation value takinginto account said observed time shift or any derivative thereof.
 5. Themethod according to claim 4, wherein said time compensation valuefurther takes into account a constant timing offset that is a functionof the base station's position.
 6. A non-transitory computer-readablemedium comprising a computer program which when run on a computer,causes the computer to carry out the method according to claim
 1. 7. Aterminal device for a non-terrestrial cellular data communicationnetwork, the terminal device comprising: a data transmission unit, adata reception unit, a memory element for storing flight trajectory dataof an airborne or spaceborne base station of said network, a memoryelement for storing terminal location data, and a processing unit,wherein the processing unit is configured to: determine at least onetime slot during which a wireless communication channel between saidterminal device and said base station is estimated to be available,based on said flight trajectory data and on said terminal location data,and to schedule a data reception or transmission between said terminaldevice and said base station during the determined time slot, whereinthe data reception unit is configured to receive during said scheduledtime slot a synchronization signal from said base station, thesynchronization signal carrying data indicating a transmissionfrequency, wherein the processing unit is further configured to computean observed Doppler shift based on a receiving frequency for saidsynchronization signal and on the transmission frequency indicatedtherein, and wherein the terminal device pre-emptively compensates saidtransmission frequency by a frequency compensation value during asubsequent data transmission to said base station, said frequencycompensation value taking into account said observed Doppler shift. 8.The terminal device according to claim 7, wherein said terminal deviceis a user equipment or a ground-based gateway node serving a pluralityof user equipment.
 9. The terminal device according to claim 7, whereinthe processing unit is further configured to, based on said flighttrajectory data, compute estimate values for said base station,comprising any of a future position, elevation, velocity, Doppler shift,Doppler drift, propagation delay, derivatives of said Doppler shift orpropagation delay, or any combination thereof, and to store theseestimate values in a memory element.
 10. The terminal device accordingto claim 7, wherein the processing unit is further configured to, basedon said flight trajectory data, compute estimate values for said basestation, comprising any of a future position, elevation, velocity,Doppler shift, Doppler drift, propagation delay, derivatives of saidDoppler shift or propagation delay, or any combination thereof, and tostore these estimate values in a memory element, and wherein saidfrequency compensation value further takes into account any of saidestimated Doppler shift values, Doppler drift, a derivative of theobserved Doppler shift, or any combinations thereof, at the time of saidsubsequent data transmission.
 11. The terminal device according to claim7, wherein the synchronization signal further carrying data indicatingtiming information, wherein the processing unit is further configured tocompute an observed time shift based on a reception time of saidsynchronization signal and on the timing information indicated therein,and wherein the terminal device pre-emptively compensates the scheduledtime of transmission by a time compensation value during a subsequentdata transmission to said base station, said time compensation valuetaking into account said observed time shift, or any derivative thereof.12. The terminal device according to claim 11, wherein said timecompensation value further takes into account a constant timing offsetthat is a function of the base station's position.
 13. A non-terrestrialcellular data communication system comprising at least one terminaldevice in accordance with claim 7, and at least one airborne orspaceborne base station for a non-terrestrial cellular datacommunication network, the base station comprising: a data transmissionunit, a data reception unit, a memory element, and a data processingunit, wherein the data processing unit is configured for transmittingdata describing a projected or actual flight trajectory of the basestation and for transmitting a synchronization signal carrying dataindicating a transmission frequency.